No moon or no axial tilt
The easiest way to make all calendars the same is to have them all be as simple as possible
The Lunar and Solar calendars don't line up. Therefore, civilizations are constantly torn between following one or the other. The months we have on our calendar are mostly based on the lunar cycles; the years on the solar cycle.
Furthermore, axial tilt causes seasons and seasons can cause interesting events to line up with certain astronomical phenomena over the world. For example, during Ancient Egyptian times, Sirius rose above the horizon right before the Nile flood started. So the calendar there gave prominence to that event.
If you have a planet with no moon and no axial tilt, then you remove most of the variation that causes calendars to be different. There will only be two astronomical phenomena of note: days caused rising and setting of the sun, and years caused by the rotation of the stars through the sky. All calendars will have the same length of days and years, with no months. For added simplicity, make the length of the year an even integer multiple of the length of a day, so that the number of days per year doesn't change over time (at last, within one person's lifetime).
Now just line up the calendar's New Year
The only problem left is that the 'start' of the calendar years must be aligned with something. If you don't care about that, then you no problem. If you do care about then you need an astronomical event of notice that occurs at the same time every year.
My proposal would be a two-year event. Have the planet and another planet be in a 1:2 resonance. Every two orbits of the inner (inhabited) planet, there will be one orbit of the outer planet. The point of conjunction would be the most obvious starting point for a New Year. Then each cultural 'year' will actually be two full orbits of the sun (how ever many days that is), starting with the conjunction with the outer planet in resonant orbit.